For the record, we use the R911 concentrate diluted and fill in the water jacket area to soak. We generally leave it for about a day.

Now the question for y'all to contemplate is what do you do about the scale which is the bigger problem? The surface rust will always be there and that is what Rust911 & Evaporust is effective at dealing with. Soaking in the bacteria does help dissolve the outer layer but it is the scale that keeps the water from dissipating/transferring the heat away from the metal. My blaster is really only effective in removing what it sees. Then the next question is how effective or clean does the block actually need to be? The bulk of the heat is carried into the head thru the chamber roof, and maybe the upper ¼ to ½ inch below the deck's surface. If you notice, the lower portion of the cylinder wall does not even have any water jackets to transfer heat into.